The present application is directed to high frequency resonant inverter circuits that operate a lamp. More particularly, the present application is directed to the resonant inverter circuit that operates continuously from an open circuit condition at the lamp's output terminals to a short circuit condition at the lamp's output terminals and will be described with particular reference thereto.
Typically, high frequency inverters use a resonant mode to ignite the lamp. The resonant mode of operation requires the inverter to operate a resonant circuit near its resonant frequency to enable the output voltage to reach sufficient amplitude, usually 2kV-3kV, to ignite the lamp. At the fundamental switching frequency, resonant mode starting causes high currents to flow through the semiconductor devices and the ballasting components. The components of the resonant circuit have to be larger and more expensive than typically needed for steady state operation. In addition, higher currents, although achieving the required output voltage, cause the inverter to dissipate more power during initial start up than during steady state operation. To reduce power dissipation, the inverter must be turned “ON” and “OFF” to reduce power dissipation.
To correct the above problems, a resonant mode at the frequencies higher than the fundamental frequency might be employed, which requires less current to flow through the inverter components. However, since a square wave is applied to the circuit that resonates at the third harmonic or higher of the fundamental switching frequency, the desired zero switching cannot be achieved. The inverter circuit might also encounter a capacitive mode of operation that would cause damage to the intrinsic diodes of the power MOSFETs. The inverter still cannot be operated continuously without excessive power dissipation in the inverter and must be pulsed “ON” and “OFF” to reduce power dissipation.
It is desirable to operate the inverter continuously without high power dissipation. The present application contemplates a new and improved method and apparatus that overcomes the above-referenced problems and others.